For CitizenIV, I'm intending to add other cities, which would be able to be flown to from the current city's airport. (the actual feature depends on me being able to write automated tools to convert at least VC and generate navigation meshes)
As I wanted that experience to be as seamless as possible, loading the game needs to be optimized to take less time. So, I took Process Monitor and looked at what happened from the point I selected to load the recently downloaded DLC (CitizenIV, again - a bug in my episode loader ) until I saw the scripts running (loading text) - in fact, the period during which the artwork loading screens appear. Out of the 18 seconds this process took on my system, around 9-10 were spent reading large chunks of data from various .img files.
I looked into the files at the offset shown by Process Monitor, and found RAGE resources of type 0x20 - collision models. I already know GTA III to SA were loading all collision meshes for various precalculation reasons during the game load, but seeing as this is around 400-500 MB of data in IV (which also has to be decompressed, as it is deflate'd, adding more time to the loading process) it takes fairly long - I wouldn't be surprised if V still does this, as it still loads slow on consoles...
As recently revealed in the mobile version of GTA:SA, Rockstar North had thought about this problem during the development of SA, and created a cache (internally named ColAccel, like my implementation is named as well) which stored the actual data read from the collision files (basically, the size of the collision models) in a cache, to load them without having to read through the full collision files. This feature never got used in SA, and clearly IV still did not have anything like that.
Well, except that I implemented something now.
ColAccel is a .asi plugin for GTA IV (without any external dependencies) which creates a 'colCache.dat' file on the first load, containing all the data actually used by the game from the collision meshes (basically, 580 kB with a lot of redundant data, nothing like 500 MB!) and accelerating your game load time by 50% or more.
Note that the plugin currently only works on GTA IV 188.8.131.52 - if demand is sufficient I'll make a version for the latest EFLC version as well.
In addition, you *should* delete colCache.dat any time you a) add additional .img files or b) replace any .wbn/.wbd files inside any .img, either directly or indirectly - if not, various bad side effects can and will happen.